[Dissociated] sensory loss is where there is SELECTIVE loss of fine touch and proprioception WITHOUT loss of pain and temperature, or vice viersa.
Caused by neurological damage by a lesion, to a single tract in the spinal cord (or brainstem)
The presentation depends on the location of the cord lesion. It can be memorized with the mnemonic that the posterior dorsal column relates to touching a girl on her back on her date, which involves touch, as well as proprioception (trying to find her back in 3D); and the anterolateral spinothalamic tract involves being karate kicked in the tummy, causing pain and hot temperature sensation. This includes:
Loss of pain and temperature is due to damage to the anterolateral spinothalamic tracts, which cross the central part of the cord close to the level where they enter it, and ascend contralaterally (i.e. travel up the spinal column on the opposite side to the one they innervate). Note that a lesion of the lateral spinothalamic tract at a given level will NOT result in a sensory loss for the dermatome at the SAME level, due to the fibers of the tract of Lissauer which transmit the neuron 1-2 levels ABOVE the affected segment, thus bypassing the segmental lesion on the contralateral side → this means that a lesion of the spinothalamic tracts will cause loss of pain and temperature below the lesion on the OPPOSITE side to it
Loss of fine touch and proprioception are due to damage to the dorsal columns, which don't cross the cord until the brainstem, and so ascend ipsilaterally (i.e. travel up the column on the same side to the one they innervate) → this means that a lesion of the dorsal columns will cause loss of touch and proprioception below the lesion and on the SAME side as it